The demand on wireless data services has grown exponentially over the last ten years, driven particularly by the popularity of smart phones. To meet this growing demand, new generations of wireless standards with both multiple input and multiple output (MIMO) and orthogonal frequency division multiple access (OFDMA), and/or single carrier FMDA (SC-FDMA) technologies have been developed, such as 3rd Generation Partnership Program (3GPP) Long Term Evolution (LTE) and Word Interoperability for Microwave Access (WIMAX). One key area is the ever growing capacity demand of the network based on these standards. One of the challenges in supporting capacity growth is the optimal usages of the limited radio resources shared by User Equipments (UEs), such as Physical Downlink Control Channel (PDCCH) usage.
Downlink Control Information (DCI) carries scheduling information for both uplink and downlink data traffic. Uplink data traffic includes data sent from a UE to a base station. Downlink data traffic includes data being sent from a base station to a UE. DCI provides a UE with the necessary information for proper reception and decoding of the downlink data transmission. There are four different DCI formats. DCI formats 0 and 3 are for uplink data transmissions and DCI formats 1 and 2 are for downlink data transmission. A DCI carrying downlink scheduling information is called a DL assignment and a DCI carrying uplink scheduling information is called a UL grant. As used herein, DL assignment may be referred to as DL assignment DCI or DL DCI. UL grant may be referred to herein as UL grant DCI or UL DCI. One UE can have one or more DCIs in a same Transmission Time Interval (TTI).
Each DCI is carried on one or multiple Control Channel Elements (CCEs) depending on the DCI length and the channel condition. The number of CCEs used, which is referred to as the CCE aggregation level, can be 1, 2, 4 or 8. All CCEs for the same DCI carry the same information. In case of multiple CCEs, i.e., higher aggregation level, the DCI payload is repeated to achieve a lower code rate, which may be needed if the UE is experiencing poor radio conditions. Each CCE consists of 9 Resource Element Groups (REG). Each REG includes 4 (or 6 in the case of a Reference Symbol) consecutive Resource Elements (RE) in the frequency domain.
A DCI is mapped to a PDCCH at the physical layer (PHY). DCIs from multiple UEs are multiplexed together in the control symbol region, which are the first few OFDMA symbols, in a TTI. The payload of the DCI is rated, matched and scrambled with a cell-specific and a slot-specific scrambling sequence. Multiple REGs from the same CCE are interleaved and cyclic shifted (CS) among different frequency and time domains to achieve good frequency and time diversity. PDCCH occupies the first 1 to 3 or 4 symbols in each TTI depending on the bandwidth.
PDCCH link adaptation (LA) is used to choose the optimal CCE aggregation level for each DCI based on radio channel conditions, i.e., channel state information, which is measured and reported to the eNodeB (eNB) as Channel Quality Indicator (CQI) by a UE. If the channel condition is good, i.e., a higher CQI, then a fewer number of CCEs or a lower CCE aggregation level may be used. If the channel condition is poor, i.e., a lower CQI, then a greater number of CCEs or a higher aggregation level will be used.
Since a maximum of 3 control symbols are used when the bandwidth is larger than 1.4 MHz (a maximum of 4 symbols in a case of 1.4 MHz), the number of available CCEs for each TTI is limited. These limited control symbols are shared by all the UEs. Therefore, the performance of PDCCH LA will greatly impact the performance of LTE Radio Access Network (RAN), such as capacity. As an example, in case of Voice over IP (VoIP), PDCCH capacity will be the key limiting factor for VoIP capacity as the demand of DCI is very high. If the PDCCH LA is too aggressive, i.e., uses less CCEs for the UE to accommodate capacity, some UEs will have more PDCCH decode failure. Then the UE cannot even locate the related DL data sent through PDSCH or UL data granted at PUSCH. This will result in significant throughput reduction and/or reduced user satisfaction level. If the PDCCH LA is too conservative, fewer numbers of users can be accommodated by the PDCCH resources, resulting in lower capacity.
According to conventional implementations, PDCCH link adaptation uses wideband CQI reports from a UE to derive wideband PDCCH Signal to Interference plus Noise Ratio (SINR). This wideband CQI is measured by the UE and reported to the eNB through uplink channels such as a Physical Uplink Shared Channel (PUSCH) or a Physical Uplink Control Channel (PUCCH). Accurate and timely reporting of the CQI by the UE helps the eNB select the optimal CCE aggregation level. This is not always the case as CQI reporting intervals are limited by signaling overhead and the accuracy may vary from UE to UE depending on UE specific implementation.
To accommodate systematic errors in CQI reporting from the UE and to track faster changes in channel conditions, a PDCCH outer-loop adjustment is normally used to generate an outer-loop adjustment, OL_ADJ, which is added to the PDCCH SINR estimation based on the CQI report. The overall estimated SINR based on the CQI report and the outer-loop adjustment is used in determining the CCE aggregation level. The outer-loop adjustment is calculated based on the PDCCH transmission result which is determined by eNB. The transmission result could be success, failure, or unknown. When the eNB cannot determine whether a PDCCH transmission is successful or not, it is referred to as “unknown” status. The outer-loop adjustment of PDCCH SINR based on different PDCCH transmission results can be calculated as follows:
If the previous transmission is a success, SINR is increased by an amount defined as UP_STEP such that the SINR adjustment is calculated as:OL_ADJ+=UP_STEP.  (Eq. 1)
If the previous transmission is a failure, SINR is decreased by an amount defined as DOWN_STEP, such that the SINR adjustment is calculated as:OL_ADJ−=DOWN_STEP.  (Eq. 2)
If the eNB is unable to determine whether the previous transmission is successful or not, i.e., “unknown,” the SINR is decreased by a fraction of DOWN_STEP, i.e., factored by a, such that the SINR adjustment is calculated as:OL_ADJ−=α*DOWN_STEP.  (Eq. 3)
The ratio of the UP_STEP and DOWN_STEP is determined based on the desired, or target Block Error Rate (BLER) percentage, i.e., the PDCCH transmission failure rate:DOWN_STEP/UP_STEP=100/BLER_TARGET−1.  (Eq. 4)
The DOWN_STEP value may be tuned to have the desired outer-loop convergence speed. The adjustment parameter a may be a tuning parameter with a value between 0 and 1. BLER_TARGET may be a predetermined value.
In the conventional PDCCH link adaptation implementation, there is one outer-loop for both DL assignments DCIs and UL grants DCIs. A single performance target in terms of PDCCH BLER applies to both DL assignment DCIs and UL grant DCIs. One outer-loop adjustment is generated and used to determine the CCE aggregation level together with PDCCH SINR. This implies: (1) that the achieved BLER of the PDCCH will be the average BLER of DL DCIs and UL DCIs. The resulting BLER for a UE's DL DCI and UL DCI may be different, but it is not possible to achieve a different BLER for DL DCIs and UL DCIs in a controlled fashion with the current, one shared single outer-loop. This also implies: (2) that the aggregation levels will be the same for DL DCIs and UL DCIs that are of the same bit length despite the possible different channel conditions that each DCI may experience. This is due to the fact that the same SINR (same CQI and same outer-loop adjustment) will be applied when selecting the CCE aggregation level
There may be situations where different PDCCH BLER targets for UL and DL DCIs may be desired. Specifically, a lower BLER may be desired for UL DCIs than for DL DCIs in several scenarios. It is may also be desirable that when DL DCI and UL DCI experience different channel conditions, which may happen due to non-perfect interleaving of the PDCCH REs and/or different interference levels, the outer-loop algorithm should be able to generate different adjustments accordingly. None of these is possible with the conventional PDCCH LA and outer-loop control.
Scenario 1
In a first scenario, it is possible that both DL and UL scheduler schedules a UL grant and a DL assignment DCIs in a same TTI. In such a case, the eNB expects that the UE sends the PDSCH acknowledgement or not-acknowledgement (ACK/NACK) corresponding to the DL assignment through the PUSCH in the same TTI that is scheduled for the UL grant.
In a case of an aggressive PDCCH DCI aggregation level when the PDCCH link adaptation and outer-loop is applied, the detection of DCI for UL grant at the UE could fail while the detection of DCI for DL assignment and detection of PDSCH could succeed or vice-versa. This may be due to certain unavoidable error rate on PDCCH transmission and a relatively slow convergence of outer-loop adjustment. In such a case, losing the UL grant impacts uplink throughput and also negatively impacts downlink throughput.
The performance differences between UL grant and DL assignment could be due to multiple reasons. One reason is that a code rate discrepancy due to the numerical error occurred during conversion from the estimated PDCCH Signal to Interference plus Noise Ratio (SINR) to the number of CCEs in each DCI. During the conversion, payload size is applied to ensure that the code rate for both DL assignment and UL grant are normalized to be the same. Initially, SINR is converted to system information (SI). SI is then converted to resource element information block (RBIR). Finally, RBIR is converted to the number of CCEs. The number of CCEs can be a decimal number, i.e., one of the following four values: 1, 2, 4, or 8. Thus, 1 CCE is assigned even if the required number is much less than 1, or only 8 CCEs are assigned when the required number of CCEs is greater than 8.
As described above, since multiple look-up tables are used and interpolations as well as rounding operations are applied to get the number of CCEs, it is likely that the code rate varies from DCI to DCI. For example, assuming DL assignment and UL grant have similar estimated SINR, with DL assignment being slightly higher, then after conversion, due to interpolation and rounding errors, DCI for DL assignment could use a CCE aggregation level 4 while DCI for UL grant could use CCE aggregation level 2.
Another reason for performance differences between UL grant and DL assignment could be due to different interference levels between UL and DL DCI. This could happen, for example, to UEs at the cell edge where Common Reference Signal (CRS) interferences from neighboring cells are strong. In shifted CRS deployment, the interference from neighboring cells will affect certain CCEs more than others. For example, if range extenders (REs) are used relatively more for UL DCI data than for DL DCI data, the decoding performance for the UL DCI data will suffer more than that of the DL DCI data.
Since the eNB expects PUSCH to carry the ACK/NACK and the UE did not send PUSCH due to the loss of the UL grant, the eNB is unsure if the PDSCH is successful or not. The eNB would then have to retransmit the previous payload, which is unnecessary. Moreover, since PDCCH OL may not converge within a few TTIs, UL grant could be lost again in the retransmission. Repeated re-transmissions impact the downlink Hybrid Automatic Repeat Request (HARQ) operation significantly and introduce many unnecessary retransmissions, resulting in reduced downlink throughput.
To solve the above described issue, the eNB can be configured to detect HARQ ACK/NACK from the same UE from both PUCCH and PUSCH even though the eNB is expecting HARQ ACK/NACK from PUSCH only. This will prevent eNB from losing the detection of HARQ ACK/NACK in PUCCH and avoid unnecessary retransmissions. The eNB will perform HARQ ACK/NACK detection from both PUSCH and PUCCH and decide which value to use. However, this solution has several drawbacks. First, extra CPU cycles will be consumed to perform ACK/NACK detection on both PUCCH and PUSCH for the same UE, which will significantly impact capacity as well as latency. Second, extra configuration signaling between media access control (MAC) and PHY is required. Third, an additional algorithm is required to reliably determine which ACK/NACK detection result from PUCCH and PUSCH will be used.
Scenario 2
In carrier aggregation (CA), multiple downlink component carriers are used and more HARQ ACK/NACKs are needed to be reported through uplink channel on primary component carrier only. So, if one UL DCI fails, multiple downlink component carriers will be affected. Therefore, it is desired to have relatively better performance in UL grant DCI compared to DL assignment DCI.
In the case of time division duplex (TDD) communication, there may be more DL TTIs that have DL assignments to this UE and the bundled and/or multiplexed HARQ ACK/NACKs are sent through PUSCH. So, if the UL grant DCI is lost, PDSCH in multiple TTIs will be impacted. Therefore, similar to CA, in TDD it is desirable to have relatively better performance in UL grant DCI compared to that in DL assignment DCI.
In both scenarios 1 and 2 above, it is desirable to be able to control the BLER targets for DL assignment and UL grant differently. Specifically, UL grant should have a lower BLER target as its impact is deemed relatively higher than DL assignment. To be able to optimally use the limited PDCCH resources when DL DCIs and UL DCIs may experience different interference levels, different CCE aggregation levels are desired for DL DCIs and UL DCIs. And since the interference levels are instantaneously changing over time, tracking the interference levels by the eNB's perception on PDCCH is difficult and tends to be slow, it is desirable to have an additional margin to ensure good performance on uplink grant.